Methods of inhibiting polymerization of unsaturated methylene compounds



Patented Oct. 21,1941

TION COMPOUNDS OF UNSATURATED METHYLENE Carl E. Barnes, Worcester,Mass., assignor, by

mesne assignments, to E. I. du Pont de Nemours a Company, Wilmington,Del., a corporation of Delaware No Drawing. Application July 8, 1939,Serial No. 283,455. In France March 1, 1939 2 Claims.

This invention relates to methods of inhibiting polymerization oicompounds containing unsaturated methylene groups.

It is accordingly an object of this invention toovercome certaindisadvantages of inhibitors previously used and to provide substancesand methods of operation which will eifectively inhibitthepolymerization of various polymerizable compounds.

A further object is to inhibit polymerization by an inhibitor which maybe readily separated from the monomer when desired or which will notprevent polymerization by feasible methods or with the aid of a suitablecatalyst capable of overcoming the inhibiting action.

Another object is to provide a method of protecting such polymerizablecompounds in their monomeric forms and thereafter polymerize the same byprocedure that is economical and simple in operation. Further objectswill be apparent in the following disclosure.

I have discovered that chromium metal will inhibit polymerization of apolymerizable monomeric acid having the formula in which R. is selectedfrom the group of hydrogen, halogen and a methylgroup, and that thisinhibiting action will tak place during storage and purification bydistillation, as well as when the monomer is used in the manufacture ofvarious products. For example, methacrylic acid containing pieces ofchromium metal may be readily distilled without polymerization takingplace. Also, the monomeric acid may be heated for a week at 100 0.without change. The other acids act similarly.

v Chromium metal will also eifectively inhibit the polymerization of themonomeric esters of monomeric acrylic acid and the alpha halogen andmethyl substituted acrylic acids provided there .is present a slightamount of one of said.monoacrylic and alpha halogen or methylsubstituted acrylic acids which is soluble in the monomer to beprotected.

The extremely low degree 0! solubility of the chromium metal in themonomeric acid and the very slight amount of acid necessary to provideinhibiting conditions for the non-acid monomers makes it possible tomaintain the monomer in a substantially pure condition. Hence, thepolymer made therefrom is not detrimentally affected, as regardstransparency, hardness, brittleness, softening point and othercharacteristics that may be desired for optical purposes or for use as abond for abrasive materials or for use as a molding compound. It is asimple procedure to remove the inhibiting metal and thus separatesubstantially all of the inhibitor from the monomer when it is desiredto polymerize the, latter. Purification by distillation is not requiredfor making various products, since polymerization of the monomer, afterthe metal has been removed, may be effected by the use of a suitablecatalyst, such as benzoyl peroxide, which needs to be used only in avery small amount to overcome the inhibitingaction of the slight amountof chromium salt that may be present in solution.

If it is desired to purify the monomeric substance, such as methacrylicacid, then the monomer may be distilled for the purpose. In that case,chromium metal is introduced into the distilling flask along with themonomer, and chromium metal or other suitable inhibitor may be placed inthe distilling column as a packing material, such as chromium granulesor other metal packing plated with chromium.

In order to distill the esters of these various acids, I may add a traceof on of the above acids along with the metallic chromium. For example,I may add methacrylic acid to the various esters of methacrylic acid andthe mono- I hydric and polyhydric alcohols, such as methyl methacrylateor other chromium salt of the and ethyl alcohols, or ethylene glycol orthe hypothetical ethylidene glycol or allyl alcohol. Crude ethylidenedimethacrylate may be distilled without polymerization by placingchromium metal only in the distilling flask.

While it is believed that all of the monomeric compounds of the groupherein described are protected by chromium metal and those salts thereofwhich form or provide compounds that are soluble in the monomericsubstance, yet it is to be understood that this invention is not to beconsidered as limited by any particular theory as to the operation ofthe inhibitor or the chemical reactions taking Place. Also, the aboveexamples and statements are to be considered as illustrative of theprinciples and specific applications of the invention and not aslimitations on the claims appended hereto.

This case is a continuation in part of my coending application SerialNo. 195,818, filed March 14, 1938: and a method of distillingpolymerizable compounds is claimed in my copending application SerialNo. 253,823, nled January 30, 1939.

I claim:

1. A polymerizable monomeric unsaturated methylene compound selectedfrom the group of the acids having the formula CHpC-C'OOH in which R isselected from the group oi hydro- 11 8811. and a methyl group, and theesters, anhydrides. halides, and nitriles of said welds. stabilizedagainst poiymerimtion by the addition thereto of a chromium salt whichis soluble in r said monomeric compound.

2. A polymerizable monomeric unsaturated methylene compound selectedfrom the group 0 the acids having the formula OHF O-C 00K

